"This study conclusively proves for the first time in the world that rat HEV can infect humans to cause clinical infection", the university added.

As noted, the rat-specific version of hepatitis E may be different than the one known to normally afflict humans, requiring scientists to keep a close watch on the situation. There was no prior evidence the rodent strain could be transmitted to humans. But viruses are said to be the most common cause around the world.

It is thought the man contracted the illness after eating food that had been contaminated by rat droppings.

Traditionally, the HEV is transmitted by the fecal-oral route, principally via contaminated water.

Hepatitis, which is characterized by inflammation of the liver, can be caused by excessive alcohol consumption, autoimmune diseases, and certain medication. To the transplanted organ, "stuck", the man used immunosuppressants.

Many people clearly have hepatitis, based on their symptoms, but they test negative on all the human strains known to exist, Adalja said. Indeed, the combination of the man's compromised immune system and exposure to rat droppings could have caused this singular case, Adalja said.

Rodent problems in Hong Kong have escalated in recent months because of a sustained spell of hot and humid weather.

Every year, the hepatitis E virus infects about 20 million people worldwide, leading to an estimated 3.3 million symptomatic cases, says the WHO.

The team was curious about how the disease crossed over from rats into a human and the human too contracted the disease from vermin infesting a refuse bin near his home.

Cases of HEV typically present in one of two ways, either as large outbreaks and sporadic cases in areas where HEV is endemic (genotype 1) in Asia and Africa, (genotype 2) in Mexico and West Africa and (genotype 4) in Taiwan and China or as isolated cases in developed countries (genotype 3), reports the WHO.

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